1. Field of the Invention
This invention is directed to improvements in paper incorporating a wide elongate impermeable element, to a method of making such paper and to documents made therefrom.
2. The Prior Art
It is generally known to include elongate security elements in security paper, as a security feature. Such elements can be threads, strips or ribbons of, for example, plastics film, metal foil, metallised plastic, metal wire. These security elements are included in the thickness of security paper to render imitation of documents produced from the paper more difficult. These elements help in the verification of security documents as they render the view of the documents in reflected light different from that in transmitted light. To increase the security provided by the inclusion of such an elongate element, it is also known to endow the element itself with one or more verifiable properties over and above its presence or absence. Such additional properties include magnetic properties, electrical conductivities, the ability to absorb x-rays and fluorescence.
As a further security feature, it has been found to be particularly advantageous to provide windows in one side of the surface of the paper, which expose such elongate elements at spaced locations. Examples of methods of manufacturing such paper incorporating security elements with or without windows are described below. It should be noted that references to “windowed thread paper” include windowed paper incorporating any elongate security element.
EP-A-0059056 describes a method of manufacture of windowed thread paper on a cylinder mould paper-making machine. The technique involves embossing the cylinder mould cover and bringing an impermeable elongate security element into contact with the raised regions of an embossed mould cover, prior to the contact entry point into a vat of aqueous stock. Where the impermeable security element makes intimate contact with the raised regions of the embossing, no fibre deposition can occur. After the paper is fully formed and couched from the cylinder mould cover, the contact points are present as exposed regions which ultimately form windows, visible in reflected light, on one side of a banknote paper.
WO-A-93/08327 describes a method of manufacturing windowed thread paper on a Fourdrinier paper-making machine. A rotating embedment means, with a modified profile for embossing, is used to drive an impermeable elongate security element into draining paper stock, on a Fourdrinier wire. The profile of the embedment means is such that raised portions are provided which remain in contact with the security element during the embedment process. Thus, paper fibres are prevented from collecting between the security element and embedment means, such that the security element is subsequently exposed in windowed regions of paper.
For production reasons, in current manufacturing procedures the security element used in windowed or non-windowed paper is preferably oscillated within the paper substrate by a small amount, for example, plus or minus 6 mm from either side of a centre line. This is primarily to assist cutting and guillotining across the thread tracks of stacks of 500 sheets. If the thread was not oscillated, the guillotine blade would be presented with a very sharply defined area, of say 1 to 2 mm wide, of polymer/metal/paper built up from 500 sheets. This tends to blunt or even chip the cutting blade. By oscillating the thread, this area is distributed over a wider area of 10 to 15 mm, easing the passage of the blade through the 500 sheet stack. The result of thread oscillation is that only parallel bars can be used in the design of the windows, if the banknotes made from the paper are all to look the same.
Recent investigations have shown that impermeable threads of 4 mm to 6 mm maximum width can be included in paper using the above methods of paper-making. This is due to the requirement for paper stock to flow around the thread and form complete paper areas on the front of the thread in the finished document.
In Canadian patent specification CA-A-2,122,528, there is described an anti-falsification paper which incorporates a wide impermeable security strip with a width between 2 mm and 4 mm. The paper is of multiply design, with at least two paper layers produced on separate paper machines. The security strip is embedded in a first ply and has perforations along the edges which permit water drainage and hence paper fibre deposition along the edges of the thread. The front of the strip is laid down over raised areas on the embossed cylinder mould cover before the raised areas enter the vat of paper stock so as to create windows of exposed strip in the contact regions. The width of the raised areas is narrower than the strip width to permit permeation through the perforations of the strip by paper fibres. However, the width of the strip is so great that the paper formed on the back of the paper has flaws in the form of arbitrary holes in the region of the strip. A second ply of ordinary paper is independently formed and the two are laminated together and further processed, the second ply thereby covering the flaws in the back of the first ply and providing at least one homogenous paper surface. In another embodiment, a third ply is laminated over the front of the first ply to wholly embed the security strip. In yet another embodiment, the width of the strip is selected to be so wide that no paper forms on the back of the first paper ply to provide a continuous exposed area on the back. The front of the strip is laid on a continuous raised area on the mould cover before the raised areas enter the vat of paper stock to provide a continuous exposed area on the front. A second ply of paper is then laminated to the first ply to form the finished security paper and give a homogenous paper layer on one side and a continuous exposed strip on the other.
In all of the prior art methods described above, the width of the elongate element which can be used is very limited. Furthermore, the areas of the threads which are exposed are restricted in terms of shape, due to the limitation imposed by the required embossings, and in terms of the areas, due to the nature of the paper-making technique itself.
WO00/39391 describes a method of making single ply paper which can have a wide strip at least partially embedded therein. This is achieved by blinding one or more selected areas of a porous support surface, depositing a first layer of paper fibres onto the porous support surface around the blinded areas, bringing an impermeable strip to lie in contact with the blinded areas of the support surface such that at least the edges of the strip overlie the deposited layer, and depositing a further layer of paper fibres over the first layer and the impermeable strip to securely embed the edges of the strip within the paper. The blinded areas are impermeable, which substantially prevents the deposition of fibres thereon before the strip is laid thereover. Thus, substantially no paper fibres are deposited on one side of the strip in a central region between edges of the strip to thereby expose a continuous area of the strip at a first surface of the paper. Additionally a plurality of discrete translucent or transparent windows are formed in a second surface of the paper in which the strip is exposed.
The preferred prior art production methods for windowed security paper require an embossed cylinder mould cover. The use of blinding is not generally used in paper-making techniques for the very reason that the paper is preferred to be uniform and consistent. The appearance of holes and perforations is not a feature which is usually required.
As the impermeable strip contacts the support surface after some paper fibres have already been deposited round the blinded areas, it prevents any further fibre deposition over the blinded areas preserving the designs created in the blinding operation as clear areas. Whilst the use of embossed cylinder moulds covers restricts the width of security threads which can be embedded, with this method it is possible to produce a document with any width of impermeable strip, from say 6 mm up to full document width, with paper “coatings” in any design, including watermarks, on the front side of the document. The back of the paper can be made to contain a continuous exposed strip which can be used for the display of indicia and the like.
However, it has been found that the windows formed by this method do not have sharp, well defined edges, but tend to be non-uniform with paper fibres encroaching into the windows and partially obscuring them. The paper fibres are not long enough to bridge the impermeable material used to blind the mould cover, but tend to mount up around the blinding material. The motion of the cylinder mould rotating in the vat of stock causes the fibre to be washed back into the hole, particularly along the leading edge.
In the method described in EP-A-0059056, however, whilst there is a limitation on the width of the security element which can be incorporated, the edges of the windows are well defined because the elongate security element makes contact with the raised regions of the cylinder mould cover before any paper fibres are deposited. Paper fibres are therefore able to enter the valleys between the raised regions so that the security element is buried in the portions of the paper known as bridges between the windows. However, it has been found that if wide elongate security elements are used in the method of EP-A-0059056, then windows are not formed when the width of the security element is wider than the raised portions of the cylinder mould cover. The edges of the security element sag over the edges of the raised regions and prevent paper fibres from entering the valleys between the raised regions, with the consequence that the security element is continually exposed on the mould cover side of the paper.
It is therefore an object of the present invention to provide an improved method of manufacturing paper incorporating a wide impermeable security element with discrete transparent or translucent windows in which the windows have clearly defined edges and are uniformly formed.